21687481 has 4 divisors (see below), whose sum is σ = 21712000. Its totient is φ = 21662964.

The previous prime is 21687473. The next prime is 21687499. The reversal of 21687481 is 18478612.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21687481 - 2³ = 21687473 is a prime.

It is a super-2 number, since 2×21687481² = 940693664250722, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21687481.

It is not an unprimeable number, because it can be changed into a prime (21687581) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 10881 + ... + 12718.

It is an arithmetic number, because the mean of its divisors is an integer number (5428000).

Almost surely, 2^21687481 is an apocalyptic number.

It is an amenable number.

21687481 is a deficient number, since it is larger than the sum of its proper divisors (24519).

21687481 is an equidigital number, since it uses as much as digits as its factorization.

21687481 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 24518.

The product of its digits is 21504, while the sum is 37.

The square root of 21687481 is about 4656.9819626020. The cubic root of 21687481 is about 278.8707965797.

The spelling of 21687481 in words is "twenty-one million, six hundred eighty-seven thousand, four hundred eighty-one".